This invention relates generally to hermetic refrigeration compressors of the type generally used in household appliances, and more particularly to a lubrication arrangement for the piston and cylinder for single piston, horizontal cylinder reciprocating compressors.
Reciprocating piston refrigeration compressors used for household appliances such as refrigerators and freezers utilize a single piston and cylinder arrangement, usually operating in a horizontal plane, and are powered by relatively low horsepower, two-pole electric motors generally in the range of a maximum of .DELTA. horsepower down to as low as 1/6 horsepower to cover the range from the smallest to the largest household refrigerators. Because of this variation in size, it is necessary for the manufacturer to provide a series of sizes for the compressors which requires not only a variation in the horsepower of the electric motor, but also a variation in the piston displacement. For reasons of manufacturing economy, while it is possible to change the displacement by varying the bore diameter of the cylinder, it is generally preferred to vary the displacement by changing the length of stroke, since this allows the use of identically machined cylinder blocks, requiring only a change in the crankshaft and piston to vary from one size to another. Furthermore, such compressors generally have a bore which is notably greater in diameter than the length of the piston stroke, not only because of a desire to keep the piston velocity relatively low but also to provide sufficient bore area for the suction and discharge valves, which are generally of the reed-type and located on a valve plate which closes off the end of the cylinder.
In order to ensure long life for the compressor, as well as to improve efficiency by reducing friction, it has long been recognized that lubrication of the moving parts of the compressor is very important. Thus hermetic reciprocating piston compressors of this type are provided with a supply of lubricating oil in a reservoir in the bottom of the casing. The bottom end of the vertical axis crankshaft extends downward into this reservoir, where it is configured to provide a pumping action either by means of the use of centrifugal force or some form of more positive displacement, such as a screw action, to force the oil in the reservoir upward through internal passages in the crankshaft to provide lubrication for the crankshaft bearings as well as the bearing for the connecting rod. Generally, in a motor up configuration, as described in the invention, excess oil is allowed to exit from the upper end of the bearing boss where it lubricates the vertical thrust bearing as well as cooling the motor before draining back into the reservoir. In an inverted (motor down) configuration, the excess oil is usually allowed to exit from the upper end of the crankshaft to spray over the interior of the compressor casing. It is also recognized that the connecting rod should be drilled to allow oil to flow from the connecting rod bearing up to the wrist pin bearing to ensure proper lubrication at this point. Further lubrication may be provided by a capillary tube having one end immersed in the oil reservoir and the other end opening into the suction plenum in the cylinder head adjacent the valve plate and the suction valve to ensure that a small amount of lubricating oil passes the suction valve into the upper end of the piston and cylinder.
Generally, it has been found undesirable to use sealing rings on the piston because of problems with increased friction and wear. Therefore, with the usual construction, the piston is carefully machined over all of the external wall surface to a very fine finish, and the tolerances of the fit in the cylinder are held to a very small amount, and this clearance is generally so low that the pistons and cylinder bores must both be carefully gauged and matched to a selected fit during assembly. This fit is sufficient to hold a thin oil film between the surface of the piston and the cylinder bore to provide adequate lubrication and to prevent any blow-by past the piston on the compression stroke.
It has therefore been recognized as desirable to provide additional oiling for the cylinder wall and the wrist pin area. While lubrication of the wrist pin can be obtained by a drilled passage in the connecting rod directly from the connecting rod bearing, various arrangements have been proposed to supply lubricating oil directly to the cylinder bore. For example, W. D. Drysdale U.S. Pat. No. 2,100,799, issued Nov. 30, 1937, shows a vertical cylinder bore compressor in which a slinger ring is mounted on the crankshaft and a pair of inclined openings are provided in the cylinder wall which become aligned with the hollow wrist pin at bottom dead center, so that oil is supplied to the interior of the wrist pin, and presumably also the cylinder wall. Other arrangements are found in horizontal cylinder compressors such as shown in W. W. Higman U.S. Pat. No. 2,286,272, issued Jun. 16 1942, and M. Y. Warner U.S. Pat. No. 2,797,857, issued Jul. 2, 1957. Both of these compressors disclose a vertical passage opening into the cylinder bore in alignment with the piston which receives oil from a reservoir formed in the cylinder block above the cylinder. In both of these patents, it is presumed that the oil will flow into the interior of the wrist pin, and thereby provide lubrication to the cylinder bore and piston.
Another cylinder bore lubrication arrangement is shown in R. M. Hvid U.S. Pat. No. 2,360,876, issued Oct. 24, 1944. This patent shows a refrigeration compressor having a single inverted vertical cylinder in which the piston is reciprocated by a connecting rod which has a ball joint connection to an eccentric rotating plate. With this arrangement, the piston not only reciprocates, but also rotates once around its axis for each reciprocation. To provide the increased lubrication required by this movement, an annular groove is formed in the cylinder wall near the midpoint of the piston at the top of the stroke, and oil is supplied to this groove through a pumping arrangement.
A more recent approach is shown in J. R. Mangan U.S. Pat. No. 2,583,583, issued Jan. 29, 1952. This compressor has a horizontal cylinder with aligned openings at the top and bottom adjacent the midpoint of the cylinder. The piston has an annular groove around its outer periphery which comes into alignment with the top and bottom openings at bottom dead center so that the oil can enter the groove from the top and the excess will flow out through the bottom opening at bottom dead center on each piston reciprocation.
While the prior art shows various arrangements for admitting oil to a portion of the cylinder bores, the oil is supplied only to limited areas of the bore and piston surfaces, and therefore optimum piston and cylinder lubrication is not accomplished.